Multistage warm air furnaces are known for use in systems for heating residential dwellings and commercial buildings. In prior art multistage warm air furnace systems a multistage thermostat has been used to call for respective lower and higher firing rates to satisfy the demand of the space being heated. The cost of multistage thermostats and the installation of same has been a consideration when installing multistage furnaces. Efforts to provide lower cost single stage thermostats for controlling multistage furnaces have not been entirely successful.
One technique for operating a multistage furnace with a single stage thermostat has been the provision of a control algorithm which is time based. The furnace is operated in an “on—off” mode with a single stage thermostat and a time based algorithm determines when the furnace is to be transitioned from a lower firing rate to a higher firing rate. This approach has certain disadvantages including noise generated by the furnace and excessive temperature swings in the controlled space. However, a single stage thermostat operating with a multistage furnace is desirable in instances where multistage furnaces are retrofitted into a building in which one or more single stage thermostats already exist. Moreover, as mentioned above, single stage thermostats are less expensive than multistage thermostats which also favors using a single stage thermostat with a multistage furnace.
Accordingly, there has been a desire to provide an improved system and method for controlling a warm air furnace, particularly a multistage furnace with a single stage thermostat, and it is to these ends that the present invention has been developed.